1. Field of the Invention
This invention relates to calibrating and compensating electronic display devices and more particularly to a method and system for automatically maintaining the uniformity of the display output of a display including organic light emitting devices (OLED).
2. Description of Related Art
Organic light emitting devices (xe2x80x9cOLEDsxe2x80x9d) have been known for approximately two decades. All OLEDs work on the same general principles. One or more layers of semiconducting organic material are sandwiched between two electrodes. An electric current is applied to the device, causing negatively charged electrons to move into the organic material(s) from the cathode. Positive charges, typically referred to as holes, move in from the anode. The positive and negative charges meet in the center layers (i.e., the semiconducting organic material), combine, and produce photons. The wavelengthxe2x80x94and consequently the colorxe2x80x94of the photons depends on the electronic properties of the organic material in which the photons are generated.
The color of light emitted from the OLED device can be controlled by the selection of the organic material. White light is produced by generating blue, red and green lights simultaneously. Specifically, the precisely color of light emitted by a particular structure can be controlled both by selection of the organic material, as well as by selection of dopants.
In a typical OLED, one of the electrodes is transparent and the cathode is constructed of a low work function material. The holes may be injected from a high work function anode material into the organic material. Typically, the devices operate with a DC bias of from 2 to 30 volts. The films may be formed by evaporation, spin coating or other appropriate polymer film-forming techniques, or chemical self-assembly. Thicknesses typically range from a few mono layers to about 1 to 2,000 angstroms.
OLEDs typically work best when operated in a current mode. The light output is much more stable and the gray scale of the device is easier to control for constant current drive than for a constant voltage drive. This is in contrast to many other display technologies, which are typically operated in a voltage mode. An active matrix display using OLED technology, therefore, requires a specific picture element (pixel) architecture to provide for a current mode of operation.
A commercially useful OLED should not only provide light output of sufficient luminosity for viewing in typical room ambient conditions but also provide a display that is uniform across the full viewing area. What this means is that each of the OLED pixels comprising the display are driven so that they all produce the same luminous output for a given input signal. The visibility of variations in the display depends on the spatial frequencies displayed in the underlying image and on the spatial frequencies in the variations. For example, relatively large errors may be tolerated in images that have high spatial frequency content. Furthermore, relatively large errors that exhibit low spatial frequency content, such as a variation that occurs gradually across an entire display, may be tolerated. Errors of this type of as much as 2% may be imperceptible to the ordinary viewer. Pixel-to-pixel errors, however, are desirably kept to less than 1%. Thus, it is desirable to control the gray scale variations in the output of individual pixels to be equal to or less than about 0.8% for most applications. As used herein, the terms xe2x80x9cpicture elementxe2x80x9d and xe2x80x9cpixelxe2x80x9d indicate both a single light emissive point and a group of closely-spaced light emissive points.
Non uniformities in pixelated display devices may be due to manufacturing non uniformities resulting in pixels with slightly different light output for the same driving current and to non uniformities due to aging of the pixels. The first type of non uniformity may be corrected with the application of a first correction coefficient that is stored in a memory and applied to the driving signal of each pixel prior to driving the pixel. The second type, however, requires continuing re-calibration of the display device during its lifetime to determine changes in pixel output uniformity. Such a process is not only expensive but oftentimes impractical.
OLED based displays are particularly vulnerable to developing time dependent uniformity changes. For example, in a display operated at a constant current density of 2.5 mA/cm2 and after an initial xe2x80x9cburn inxe2x80x9d time of about 100 hours, the light output of the OLED decays from 150 cd/m2 to 110 cd/m2 after 3000 hours of operation, where operating voltage increases from 3.1 to 4.1 Volts. Because the luminous efficiency of a pixel varies with the total amount of light it produces, adjacent pixels in a display may age differently. Thus, an initially calibrated uniform display may develop non-uniformities over time, which depend on the driving history of each pixel. These non-uniformities may require periodic optical calibration to maintain a uniform display. Other types of emissive displays and transmissive displays may also develop non-uniformities due to long-term differences in the activation of pixels. If for example, the image on an initial input screen is displayed when a computer monitor is not in use for a prolonged period of time, for example, overnight for several months, that image may persist on the display device even when all image pixels are driven to what should be a uniform value. This type of persistent image may occur on cathode-ray tubes, field-emissive displays, electroluminescent displays and liquid crystal displays.
Additionally, determining whether a display is uniform is not always an easy proposition, because as was stated earlier, in the best conditions, an observer can detect intensity variations of only 0.8% or more. There is therefore needed not only for a method to rapidly and accurately correct resulting non uniformities of an initially calibrated display during its life, but a method for measuring such uniformities with better accuracy than the accuracy provided by visual observation in a manner that is easy to implement.
The present invention is embodied in a method and associated system that calculates and predicts the decay in light output efficiency of each pixel beginning from a starting measured level based on actual integrated drive current applied to each pixel and derives a correction coefficient that is applied to the next drive current for each pixel.
In one exemplary embodiment of the invention, the calculation is based on the following equation that predicts the current needed at a present period to produce the same output as in a previous period:
IN=INxe2x88x921 exp[INxe2x88x921xcex94tNxe2x88x921/Ioxcfx84o].
In this example, Io is the initial condition and xcfx84o is the corresponding delay time, which may be measured during an initial xe2x80x9cburn-inxe2x80x9d interval. The value of Io is preferably determined after the burn in interval and after the calibration of the light output of an OLED panel using, for example, a CCD camera to provide an output signal indicative of the light output of the OLED panel that is substantially the same for each individual pixel of the display panel and substantially constant across the full panel.
In another exemplary embodiment of the invention, the calculation is based on an instantaneous current-voltage characteristic of the image pixel. The difference in voltage across the pixel needed to produce a predetermined current is measured and is used to index a table of stored values, the stored values indicate a current level that provides a desired brightness in the displayed pixel.
The present invention also provides a system that corrects non uniformities in the light output of an electronic display device including a plurality of addressable discrete picture elements (pixels), each of the pixels driven by a driving current and each pixel having a light output that is a function of the driving current. The system includes:
a) an accumulator that integrates the driving current for each of the pixels during the elapsed time;
b) circuitry responsive to the integrated current value for calculating a corrected driving current,
b) correction apparatus for applying the corrected current to each of the plurality of pixels.
The present invention further provides a method for calibrating a display device comprising an array of individually adjustable discrete picture elements (pixels) using a radiation sensor that may be a single radiation sensing device or using a camera comprising an array of radiation sensing devices, the method comprising:
a) observing with the radiation sensor a first area of the display device array forming a first level sub-array comprising a first number of pixels and adjusting each of the pixels within the first sub-array to a desired light output;
b) observing with the radiation sensor a second area forming a first level second sub-array and again adjusting each of the pixels within the second sub-array to the desired light output;
c) repeating steps (a) and (b) until all of the display pixels have been adjusted to the desired output.
According to one aspect of the invention, the method further includes the steps of:
d) observing with radiation sensor another first area of the device array containing a plurality of the first level sub-arrays to form a second level sub-array;
e) adjusting as a unit each of the first level sub-arrays in the second level sub-array, to the desired output;
f) observing, with the radiation sensor, another second level sub-array containing a plurality of the first level sub-arrays to form an other second level sub-array;
g) adjusting as a unit each of the first level sub-arrays in the other second level sub-array, to the desired output;
h) repeating steps (e) through (g) until all of the display first level sub-arrays have been adjusted to the desired output;
i) repeating steps (e) through (h) with successively larger sub-arrays until the sub-arrays reach the size of the display array.